The present invention relates to apparatus for freeze-drying particles of material, of the kind which has a rotary sublimating enclosure equipped with agitator or tumbler vanes which normally operate about a horizontal axis.
Freeze-drying apparatus is already known having a sublimating enclosure fitted with a frusto-conical set of vanes, the vanes each being adapted to form catchment troughs when they passed through a lower zone of the said enclosure and to form chutes emptying into an axial discharge area when they ascended. In this way, when it was desired at the end of sublimation to extract the particles automatically, it was sufficient to open a downwardly inclined discharge passage which began at the said axial zone, preferably by placing a discharge deflector in the path of the particles as they flowed from the said vanes towards the axial area. An apparatus of this type has undeniable advantages, due to the fact that it is possible to discharge the particles automatically at the end of the freeze-drying process, which, if required, allows them to be kept under vacuum or in a controlled atmosphere. A completely cylindrical freeze-drying enclosure will not allow discharge by this method. However, agitating the particles during sublimation by means of a frusto-conical set of vanes of the kind described above has the drawback that the agitation is too violent, being far more vigorous than that from a cylindrical set of vanes, and results in the formation of "fines", which are particularly undesirable in certain applications.
It is an object of the invention to provide a freeze-drying method and apparatus which enable the advantage of automatic discharge achieved with a frusto-conical set of vanes to be combined with that of the gentle and homogeneous agitation achieved with a cylindrical set of vanes.